The invention relates to porous polymers and solid-state expansion processes using solvents. The processes can be used to make porous films, fibers, tubes, and coatings for use in filters, chromatography and numerous other applications.
Porous semicrystalline polymers have a range of important and useful applications. In typical applications, the control of pore structure and purity of the product, bulk mechanical properties, and macroscopic shape are of fundamental importance.
Porous semicrystalline polymers can be produced by crystallization from solution. In the well-known process of thermally induced phase separation (TIPS), the porous material is formed from homogeneous solution by lowering the temperature, inducing crystallization, and/or liquid-liquid phase separation. The TIPS method involves dissolving a polymer in a solvent. The solid product forms from solution, either assuming the shape of the crystallization vessel, or becoming film or sediment at the bottom of the vessel. This method cannot be used to create complicated shapes (e.g. complicated injection molded parts). Complete removal of solvent (e.g., drying) is generally difficult (often a second solvent is used to extract the first solvent) and the surface forces of the solvent can lead to pore collapse during removal. Problems associated with current methods include: inability to control fine pore structure and pore size distribution, lack of mechanical coherency in the product, reliance on hazardous processing solvents, and solvent removal and recovery from the final product. Other methods such as foaming, sintering, stretching, and leaching have also been developed over the years to create porous materials with desired properties.
Increasingly strict environmental legislation has forced many industries to reevaluate their use of hazardous solvents. International agreements such as the Montreal Protocol (1987), the Clean Air Act Amendments (1990), and the Kyoto Summit (1997) have all had as their focus the reduction or elimination of volatile organic compound (VOC) emissions as a way to stop ozone depletion and greenhouse warming. The polymer industry in particular is notorious for its reliance on VOCs, which have been used as monomers, solvents, plasticizers, and cleaning agents in polymer synthesis and processing.
The invention is based on the discovery that crystallizing constrained polymers from swollen states can lead to porous structure, including open celled, bicontinuous porous structure. Solvents can include supercritical fluids (SCF). After crystallization, from the swollen state the polymers show an increase in volume, a decrease in density, and the overall shape is controlled by the shape before swelling and the processing history. Scanning electron micrographs of the samples show an open cell porous, network.
In one aspect, the invention provides a new process for creating porous polymers, the pore structure and distribution of which can be controlled through material properties and processing parameters. The process is applicable to many different types of polymers. The final shape of the porous polymer is determined by shaping methods such as extrusion, blow molding, fiber spinning, and injection molding applied prior to the process, as well as by material properties, and further processing history.
In another aspect, the invention provides porous polymeric materials with open pore structures having new morphologies, improved pore size distribution, and improved mechanical strength. These porous polymers are produced in such a way that all interior surfaces are extremely clean, and do not contain residual materials (such as residual solvents, for example) which are typically introduced by previously used processes. This property can reduce or eliminate the need for post-processing cleaning, and can make the porous polymers amenable to further processing such as surface modification, surface functionalization, or biological and medical applications.
The invention, in some embodiments, further provides porous materials of increased strength, by virtue of a crosslinked structure. The shaping of polymers before processing is also substantially maintained during processing, which results in porous materials having a wide variety of shapes that were previously unavailable.
In one aspect the invention provides a method for producing porous structure in a polymer. The method includes shaping a polymer; constraining the structure of at least a portion of the polymer; melting the polymer; contacting the melted, constrained polymer with a solvent under conditions, and for a time sufficient to cause at least partial swelling of the polymer; crystallizing the swollen polymer, and removing the solvent, to yield a porous polymer. The solvent can be a supercritical fluid, such as propane. Some of the steps can be performed simultaneously. The shaping can be by reactive extrusion. The structure of at least a portion of the polymer can be constrained by crosslinking, for example, as achieved by radiation, by reacting functional groups on the polymer, by chemical radical-initiation, or by photochemical reaction. The method can also include extracting an uncrosslinked portion of the polymer from the crosslinked portion of the polymer with a solvent before crystallization to produce a solution comprising an uncrosslinked portion of polymer. This method can also include extracting substantially the entire uncrosslinked portion of the polymer from the crosslinked polymer, and can also include impregnating the crosslinked portion of the polymer with a further material, wherein the further material penetrates the interior of the crosslinked portion of the polymer, and can also include impregnating the crosslinked portion of the polymer with a further material, wherein the further material remains substantially on the exterior of the crosslinked portion of the polymer. The further material can include a polymer, a cell culture, a pharmaceutically active material, a lubricant, or a reactive crosslinking material. The method can also include replacing the solution comprising uncrosslinked portion of polymer with solvent containing substantially no uncrosslinked portion of polymer.
In another aspect, the invention provides a method for making a shaped material. The method includes allowing a solidifiable material to impregnate the interior of a porous structure; solidifying the solidifiable material; and removing the porous structure to produce a shaped material. The porous structure can have pore sizes between about 0.01 xcexcm and 100 xcexcm. The solidifiable material can be an inorganic sol, such as a metal alkoxide or metalloid alkoxide.
In another aspect, the invention provides a porous crosslinked polymer having pore diameters from about 0.01 xcexcm to about 100 xcexcm, and having a open-cell, bicontinuous structure. This porous crosslinked polymer can form part of a tissue scaffold, a catalyst substrate, a liquid or gas filter.
In another aspect, the invention provides a method for growing cells including providing a porous crosslinked polymeric scaffold; at least a portion of the surface of which is coated with cells; and allowing the cells to grow for a time, and under conditions, sufficient to produce new cell. The cells produce a material excreted into an extracellular matrix, or the cells and new cells form tissue.
In another aspect, the invention provides a battery separator comprising a porous crosslinked polymer.
In another aspect, the invention provides a porous polymer having pore sizes between about 0.1 xcexcm and 100 xcexcm. The volume porosity can be of from about 1% to about 90%, and can have an open-celled, bicontinuous pore structure.
The invention provides a number of advantages. The process is markedly simpler and more cost efficient than previous methods. Standard polymers can be employed in the process, rather than only high cost specialty polymers required of prior processes. The shaping of the polymer can be carried out in steps separate from pore generation. The variety of shapes and relative dimensions possible (thin, thick, round, flat, surface coating, bulk material) is greater than that enabled by prior processes. The product is substantially clean, and can be readily used in medical applications. The new process avoids the use of hazardous or environmentally damaging solvents, for example those with volatile organic components. The solvent can be recovered in a useable form after use, and solvent use is thereby reduced. The new processes described herein are ideal for high yield and large scale production, such as reactive extrusion. The processes are useful for inexpensive commodity polymers, and can replace foaming or TIPS.
The polymeric products have a regular pore structure, and can be used in separations as filters, membranes, or chromatography support. The high surface areas of open cell networks make them ideal candidates for catalyst supports where high surface area-to-volume ratios are crucial. Biomedical applications of open cell networks include scaffolding for tissue growth and controlled-release drug delivery methods. Other applications include textiles having xe2x80x9cbreathablexe2x80x9d laminates or fibers, porous nonwoven materials, thermal insulation material able to pass water vapor, porous precursors for forging (prosthetic devices) or fiber spinning (with pore modifications possible through stretching, with high modulus final products available by ultradrawing), low dielectric coatings for electronic parts and wires, filter applications for liquid purification, membrane separators for gas and liquid separations, support for transport media (such as battery separators, and fuel cell membranes), super absorbing linings for diapers and the like, semi-permeable vesicles or bottles (for controlled release), introducing porosity into closed cell porous materials such as foams, templates for producing inorganic porous materials, and other applications. The articles produced according to these methods take on a wide variety of shapes including films, sheets, tubes, fiber, and bulk forms.
As used herein, xe2x80x9cbicontinuous porous structurexe2x80x9d refers to pore structures in materials in which a continuous path can be traced through either the pore voids or the pore walls across the material.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.